.weilei {
    padding: 20px;
    color: white;
    background-color: green;
    margin: 10px;
    width: 20%;
    text-align: center;
}
.weilei:hover {
    background-color: blue;
}
.weilei2:hover p {
    /* 鼠标悬停到这里显示P元素  .weilei2:hover p  */
    display: block;
}
.weilei2 p {
    padding: 20px;
    background-color: yellow;
    /* 先不能让p元素显示出来； */
    display: none;
    width: 30%;
    text-align: center;
}

/* ???? */
body > p:first-child {
    color: blue;
}

p i:first-child {
    color: red;
}
p:first-child i {
    color: aquamarine;
}
p:lang(en) {
    quotes: "~" "~";
}
input:focus {
    background-color: yellow;
}
/* first-line只适用于块级元素 */
.first-line::first-line {
    color: rebeccapurple;
    font-family: 'Courier New', Courier, monospace;
    background-color: azure;
    word-spacing: 2px;
    text-decoration: none;
    line-height: 30px;
}
.first-line::first-letter {
    color: red;
    font-size: 50px;
}
h2::before{
    /**/
    /* 怎样设置这个链接里面的图片大小？*/
    content: url(../images/绿草茵茵.jpg);
    /* width: 100px;
    height: 100px; */
}
h2::after {
    content: url(../images/翱翔天际.jpg);
}
/**/
/*selection伪元素匹配用户选择的元素部分*/
.selection::selection {
    color: red;
    background-color: yellow;
}
.opacity {
    opacity: 0.5;
}
.opacity:hover {
    opacity: 1;
}
.four {
    background-color: #4CAF50;
    padding: 10px;
}
div.four {
    opacity: 1;
}

div.first {
    opacity: 0.1;
}
div.second {
    opacity: 0.3;
}
div.third {
    opacity: 0.6;
}

.background {
    content: url(../images/绿草茵茵.jpg);
    border: 10px solid gray;
}

.transbox {
    margin: 30px;
    opacity: 0.5;
    border: 1px solid gray;
    width: 50%;
    height: 50%;
    text-align: center;
}
  /*为何这个p段落里面的字不能显示出来呢？*/
.transbox p {
    margin: 10px;
    font-weight: 100px;
}
ul {
    /* display: none; */
    list-style-type: none;
    margin: 0;
    padding: 0;
    /* border: 1px solid black; */
}
/*
display:block; 可以将链接显示为块元素，可以使整个文本都被单击（而不仅仅是文本），还可以指定宽度和高度等等；
**/
li a {
    background-color: rgb(215, 212, 212);
    display: block;
    padding: 10px;
    font-size: large;
    width: 200px;
    text-decoration: 200px; /*文本下划线去掉*/
    text-align: center;
    border-bottom: 1px solid black;

}   
li:last-child{
    border-bottom: none;
    /* background-color: red; */
}
li a:hover {
    background-color: gray;
    color: white;
}
/* first-child 没有选中？？*/
/* li:first-child {
    background-color: rgb(103, 103, 198);
} */
li a.active {
    background-color: green;
    color: white;
}
